An unexpected ally in understanding our brain

Eirinet Gómez

La Jornada Newspaper, Tuesday, November 11, 2025, p. 6

The leech has become an unexpected ally in understanding how the human brain works. Its neurons, which share similar mechanisms and genes with ours that have been maintained throughout evolution, allow us to observe live how serotonin, a key neurotransmitter that regulates mood, sleep, emotions, and attention, is released.

José Arturo Laguna Macías, a doctoral student in biomedical sciences at the Institute of Cellular Physiology of UNAM, explained that through these invertebrates they have been able to study step by step the complex process by which neurons communicate and better understand how brain activity is organized.

In an interview with La Jornada , he explained that they used leeches in this research because they share small, functional "parts" in common, such as ion channels that allow the passage of molecules, calcium sensors, and vesicle fusion machinery, among others.

Serotonin release

The leech's nervous system, unlike ours and that of mammals, is segmented into 21 ganglia connected by nerve cords that run from the animal's head to its tail, like a string of beads. Each ganglion contains 400 neurons with a stereotyped distribution, making it easy to distinguish a pair of large, serotonergic Retzius neurons (named after their discoverer, Gustaf Retzius).

“These neurons are ideal for observing how serotonin is released from the soma (the neuron's body) because we can extract them and keep them in culture, stimulate them, record their activity, and inject solutions while observing them under a microscope.”

Initial laboratory work has allowed them to map this release pathway from the soma and its key components, which depends on calcium and requires the mobilization of its components. Lagunas Macías is now focused on identifying the proteins that execute the release of serotonin from the somatic membrane.

“Proteins are like tools that the cell manufactures from a gene, and each one performs a specific task, for example, detecting calcium, moving vesicles, joining membranes. The next step is to move from the level of tools to that of instructions: to find out which genes and which signaling pathways coordinate each stage of the process and when they are turned on or off in response to different signals,” he explained.

Defining this type of neuronal communication, the researcher mentioned, allows us to understand how the brain regulates its state and perceives the world.